Singular | Plural
------- | --------
child | children
tooth | teeth
foot | feet
mouse | mice
man | men
woman | women
person | people
goose | geese
ox | oxen
sheep | sheep
fish | fish (or fishes for different types)
deer | deer
series | series
species | species
datum | data
medium | media
phenomenon | phenomena
radius | radii
crisis | crises
basis | bases